If you know the answers to 50% of the questions, can narrow down 25% of the questions to a 50-50 chance of getting it right and finally make wild guesses on the remaining 25%. Your total score should be 50 + (25*0.5) + (25*0.25) = 68.75% This should be very close to a pass mark.

And if you have a 100% certainty of getting 50% of the CFA questions correct then you should pass the exam. That 100% certainty looks pretty suspect to me.